The making of polyolefin packaging films is well-known and includes the techniques of casting films as thin sheets through narrow slit dies, and the blownfilm techniques wherein an extruded tube of molten polymer is inflated to the desired "bubble" diameter and/or film thickness. In an exemplary process, the resins and additives are introduced into an extruder where the resins are melt plastified by heating and then transferred to an extrusion die for formation into a film tube. When desired, films may then be crosslinked by various means, with electron beam crosslinking preferred.
In an electron beam processor (EBP), electrons are generated and accelerated in a vacuum then emitted to irradiate an object with electrons. The electron beam processor is used in various applications including curing of coatings and inks, irradiation of foodstuff, sterilization of medical products, as well as crosslinking and grafting of polymeric materials.
An electron beam processor typically comprises: a) a power supply, b) an electron emitter, e.g. a heated cathode, c) an accelerator for shaping the emitted particles into a beam and for directing and accelerating the energized particle beam towards an emissions window, d) a vacuum chamber from which air molecules are removed so air cannot interfere with the generation of the particle beam, e) a support structure which provides a method to seal the vacuum chamber, f) an emissions window, g) a product treatment area where the object being irradiated passes through, and h) an emission collector plate. Electron beam energy is expressed by the acceleration voltage, which is typically in the range of 100 kV to 10,000 kV. A polyethylene crosslinking application typically runs at high energy levels with typical voltages over 500 kV.
The use of multi-pass irradiation to treat polymer films is generally known. Typically this is done by passing the film over a series of rollers and irradiating at a high voltage. U.S. Pat. No.3,126,680, Baird, Jr. et al, discloses such a process, indicating that the voltage should be high, 750 kV or higher, preferably at least 1000 kV. Though this method may be preferred for irradiating polymer films, the electrons must pass through not only two or more thicknesses of the film, but also through the intervening air spaces. This typically has not been an impediment provided the voltage of the machine, or the energy of the electrons, is high enough. (James H. Bly, Electron Beam Processing, 106-107 (1988).)
Multi-pass irradiation of polymer films at low voltages requires special considerations. Applicants have designed an apparatus and process to overcome the problems of low voltage, multi-pass irradiation.